I think variables for more than width and height might actually be a bit redundant, since all the other variables are just copying over values from src, which are known values when the tag is being created, that won't change on their own. A developer could just type them a second time and be done with it. This doesn't prevent the width and height variables from being used as part of a folder structure or filename either. Trying to use a something like {filename} could also cause problems if you want to use src="img/blog/anImage_200x300.jpg" as your fallback, unless the variable can strip the sizes out on it's own.
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Mike Gossmann